Isomerization of 2-pentenoates to 3-pentenoates

ABSTRACT

3-Pentenoates are prepared by isomerization of 2-pentenoates to 3-pentenoates by a process in which the 2-pentenoate is treaated with from 0.01 to 0.5 mole of cyclic tertiary amine having a pK a  of &gt;6 per mole of 2-pentenoate at from 50° to 250° C. and the 3-pentenoate is distilled off continuously.

J. Org. Chem. 33 (1968), 1671 et seq. discloses that 2-pentenoates canbe isomerized photochemically to 3-pentenoates. A disadvantage of thisprocedure is that it requires a very long irradiation time in order toachieve adequate yields. Furthermore, it is necessary to use very dilute2-pentenoate solutions, which are expensive to work up.

It is also known that 2-pentenoates can be converted to 3-pentenoates bymeans of organometallic bases, such as lithium diisopropylamide (Helv.Chim. Acta 64 (1981), 1023 et seq.) and potassium disilazide(Tetrahedron Lett. 25 (1984), 5181). A particular disadvantage, however,is that the organometallic bases are very sensitive to water,necessitating a very careful procedure under a protective gasatmosphere. Moreover, these bases have to be used at very lowtemperatures, which is technically very complicated.

According to Can. J. Chem. 46 (1968), 2225 et seq., 2-pentenoates can beconverted to 3-pentenoates by a purely thermal method at above 250° C.However, this requires considerable reaction times, for example morethan 150 hours, in order to achieve an adequate conversion.

J. Org. Chem. 47 (1982), 2745-2748 furthermore describes theisomerization of pentenoates with equimolar amounts of1,8-diazabicyclo[4.5.0]undec-7-ene. However, it is not stated how3-pentenoates can be obtained from 2-pentenoates.

It is an object of the present invention to provide a process in which2-pentenoates are converted as completely as possible to 3-pentenoatesat relatively low temperatures and in a short time, 4-pentenoates, whenpresent, as far as possible not being converted.

We have found that this object is achieved by a process for thepreparation of a 3-pentenoate by isomerization of a 2-pentenoate,wherein a 2-pentenoate is treated with from 0.01 to 0.5 mole of acyclic, tertiary amine having a pK_(a) >6 per mole of pentenoate at from50° to 250° C., and 3-pentenoate is distilled off continuously.

The novel process has the advantages that 2-pentenoates can besubstantially converted to 3-pentenoates, the process takes place in arelatively short time and does not require any expensive catalysts, andvirtually no isomerization of 4-pentenoates occurs.

The novel process is noteworthy in that Japanese Patent Publication56-55345 discloses that 3-pentenoates undergo up to 60 mol %isomerization to 2-pentenoates in the presence of1,8-diazabicyclo[5.4.0]undec-7-ene at 100° C. This is in agreement withthe well known fact that conjugated double bonds are thermodynamicallyfavored over isolated double bonds. Also noteworthy is the fact thatmethyl 2-trans-butenecarboxylate undergoes 61% conversion to methyl2-cis-butenecarboxylate and only 9% conversion to methyl vinylacetatewhen treated with 1,8-diazabicyclo[5.4.0]undec-7-ene with continuousdistillation of buteneesters at 118°-120° C.

The 2-pentenoates used as starting compounds are preferably derived fromalkanols of 1 to 12 carbon atoms, cycloalkanols of 6 to 8 carbon atoms,aralkanols of 7 to 10 carbon atoms, phenols or naphthols. Examples ofsuitable starting materials are the corresponding methyl, ethyl,n-propyl, isopropyl, tert-butyl, isobutyl, n-butyl, sec-butyl, n-pentyl,n-hexyl, dodecyl, cyclopentyl, cyclohexyl, cyclooctyl, benzyl,phenylethyl and phenyl esters. The 2-pentenoates used as startingmaterials may be in the cis and/or trans form. Alkyl 2-pentenoates, inparticular C₁ -C₃ -alkyl 2-pentenoates, are particularly suitable.

The treatment is carried out at from 50° to 250° C., particularlyadvantageously from 100° to 160° C.

As a rule, the treatment is effected under atmospheric pressure,although it is also possible to employ reduced pressure, for example upto 1 mbar, or slightly superatmospheric pressure, for example up to 10bar.

The treatment is carried out in the presence of a cyclic, tertiaryamine, having a pK_(a) of >6, in particular from 6.2 to 12.

Preferred cyclic, tertiary amines are

(a) 5-membered to 7-membered, cyclic, tertiary amines which may contain1 or 2 nitrogen atoms or 1 nitrogen atom and 1 oxygen atom in the ring,one or both nitrogen atoms being substituted by alkyl of 1 to 4 carbonatoms. Examples of suitable compounds are N-methylpyrrolidine,N-methylpiperidine, N-methylhexamethylene imine, N-ethylmorpholine andN,N'-dimethylpiperazine. 5-membered to 7-membered, tertiary, cyclicamines having only one nitrogen atom in the ring are particularlypreferred.

(b) Aminopyridines of the formula I ##STR1## where R₁ and R₂ areidentical or different and are each alkyl of 1 to 4 carbon atoms or,together with the nitrogen atom on which they are substituents, may forma 5-membered to 7-membered ring which may additionally contain an oxygenor nitrogen atom. Particularly preferred compounds of the formula I arethose which are derived from 4-aminopyridine. Examples of suitablecompounds are N,N-dimethyl-4-aminopyridine, N,N-diethyl-4-aminopyridine,4-morpholinopyridine and 4-N-piperazinopyridine.

(c) Imidazoles of the formula ##STR2## where R₃ is alkyl of 1 to 4carbon atoms. Examples of suitable compounds are N-methyl-, N-ethyl- andN-propylimidazole.

(d) Bicyclic amidines, in particular 1,8-diazabicyclo[5.4.0]undec-7-ene,1,5-diazabicyclo[4.3.0]non-5-ene and 1,4-diazabicyclo[2.2.2]octane.

It is of course also possible to use mixtures of the abovementionedcyclic, tertiary amines.

From 0.01 to 0.5, in particular from 0.05 to 0.2, mole of cyclic,tertiary amine is used per mole of 2-pentenoate.

The 3-pentenoates formed during the isomerization are distilled offcontinuously, from the reaction mixture, preferably at the rate at whichthey are formed. For this purpose, the bottom product is heated at theboil, advantageously while continuously feeding in the 2-pentenoate, andthe resulting 3-pentenoate is distilled off continuously, preferably atthe rate at which it is formed.

It has proven particularly useful to use cyclic tertiary amines whichhave a boiling point above that of the particular 3-pentenoate formed sothat, when 2-pentenoate is fed continuously into the reaction mixture,the cyclic tertiary amine remains behind and only 3-pentenoates areremoved from the reaction mixture. If the 3-pentenoate thus obtainedalso contains residual amounts of 2-pentenoate, the latter can beseparated off in a second distillation and are preferably recycled tothe isomerization stage.

Compared with the processes known to date, the novel process has theadvantage that the 2-pentenoates are converted to 3-pentenoates rapidly,under mild conditions and with high selectivity. Other advantages arethe possibility of recovering the nitrogen bases and the fact that thereaction mixtures can be worked up easily. It is also noteworthy that4-pentenoates present at the same time remain virtually unchanged. Thisis important since, for example, methyl 2-cis- and 4-pentenoate havevirtually the same boiling point and therefore cannot be separated fromone another by distillation.

The 3-pentenoates obtainable by the process of the invention areimportant intermediates for the preparation of adipic acid byhydroesterification.

The Examples which follow illustrate the process according to theinvention.

EXAMPLE 1

A mixture of 684 g of methyl 2-trans-pentenoate (boiling point 137°C./1013 mbar) and 73 g of 4-N,N-dimethylaminopyridine was heated to theboil in a 1 distillation flask having an attached packed column (height2 m, diameter 36 mm, 2.1 of wire coils) and a reflux divider. Apentenoate mixture having a boiling point of 127°-136° C./1013 mbar wastaken off at the top of the column (reflux ratio 6:1). Correspondingamounts of methyl 2-trans-pentenoate were metered into the bottom of thecolumn. In the course of 30 hours, 3,936 g of pentenoate mixture wereremoved from the top of the column, the said mixture consisting of 8% ofmethyl 2-cis-pentenoate, 14% of methyl 2-trans-pentenoate and 78% ofmethyl 3-cis- and 3-trans-pentenoate.

EXAMPLE 2

A mixture of 661 g of methyl 2-trans-pentenoate, 68 g of methyl2-cis-pentenoate and 73 g of 4-N,N-dimethylaminopyridine was heated tothe boil in the distillation apparatus described in Example 1. Apentenoate mixture having a boiling point of 125°-134° C./1,013 mbar wastaken off at the top of the column (reflux ratio 6:1). Correspondingamounts of a pentenoate mixture consisting of 90% of methyl2-trans-pentenoate and 10% of methyl 2-cis-pentenoate were metered intothe bottom of the column. In the course of 6.5 hours, 704 g ofpentenoate mixture were removed from the top of the column, the saidmixture consisting of 12% of methyl 2-cis-pentenoate, 12% of methyl2-trans-pentenoate and 76% of methyl 3-cis- and 3-trans-pentenoate.

EXAMPLE 3

17.1 g of a pentenoate mixture which consisted of methyl2-cis-pentenoate (20%), methyl 2-trans-pentenoate (40%), methyl 3-cis-and trans-pentenoate (20%) and methyl 4-pentenoate (20%) and 0.95 g of1,5-diazabicyclo[4.3.0]non-5-ene were heated to 130° C. The mixture ofthe pentenoates was separated from the catalyst by distillation in abulb tube apparatus (60°-100° C./20 mbar). Analysis by gaschromatography showed that the distillate (16.5 g) consisted of 2.8% ofmethyl 2-cis-pentenoate, 48.7% of methyl 2-trans-pentenoate, 27.9% ofmethyl 3-cis- and trans-pentenoate and 19.9% of methyl 4-pentenoate.

EXAMPLE 4

A mixture of 15 g of methyl 2-cis-pentenoate and 0.2 g of1,8-diazabicyclo[5.4.0]undec-7-ene was heated to 130° C. After 4 hours,the pentenoate mixture formed was separated from the catalyst bydistillation in a bulb tube apparatus. 14.8 g of distillate and 0.3 g ofresidue were obtained in this procedure. The distillate consisted of 7%of methyl 2-cis-pentenoate, 53.2% of methyl 2-trans-pentenoate and 39.6%of methyl 3-cis- and transpentenoate.

EXAMPLE 5

A mixture of 20 g of methyl 2-cis-pentenoate and 2.1 g ofN,N-4-dimethylaminopyridine was heated at 180° C. for 1 hour in a glassautoclave. After this time, analysis by gas chromatography indicated27.5% of methyl 2-cis-pentenoate, 33.7% of methyl 2-trans-pentenoate and38.3% of methyl 3-cis- and 3-trans-pentenoate. Methyl 3-pentenoate wasthen distilled off continuously from the mixture.

EXAMPLE 6

A stirred mixture of 20 g of methyl 2-cis-pentenoate and 2.2 g of1,5-diazabicyclo[4.3.0]non-5-ene (DBN) was heated at 130° C. for 0.5hour. After this reaction time, analysis by gas chromatography showedthat the reaction mixture consisted of 3.2% of methyl 2-cis-pentenoate,61.3% of methyl 2-trans-pentenoate and 34.2% of methyl 3-cis- and3-trans-pentenoate. Methyl 3-pentenoate was then distilled offcontinuously from the reaction mixture.

When this experiment was repeated under the same conditions with methyl2-trans-pentenoate, the reaction mixture had the following compositionafter a reaction time of 0.5 hour: 3.1% of methyl-2-cis-pentenoate,61.1% of methyl 2-trans-pentenoate and 35.7% of methyl 3-cis- and3-trans-pentenoate.

EXAMPLE 7

When Example 6 was repeated using 2.7% of1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) instead of DBN, the reactionmixture had the following composition after 0.5 hour at 130° C.: 3.5% ofmethyl 2-cis-pentenoate, 63.9% of methyl 2-trans-pentenoate and 32.6% ofmethyl 3-cis- and trans-pentenoate.

EXAMPLES 8 TO 12

Examples 8 to 12 were carried out in a similar manner to Example 5. Thetype of pentenoate used, the N base, the reaction time and thecomposition of the reacted mixture are summarized in Table 1.

    __________________________________________________________________________                                PGC (% by                                                      Molar     Reaction                                                                           area,                                                PAE.sup.1                                                                         Base  ratio Temp.                                                                             time 2-cis-                                                                           2-trans-                                                                          3-                                         No.                                                                              used                                                                              used  PAE:base                                                                            [°C.]                                                                      [h]  PAE                                                                              PAE PAE                                        __________________________________________________________________________    8  2-cis-                   52 6   40                                         9  2-trans-                                                                          DABCO.sup.2 135 4    2  72  22                                         10 2-cis-    1:0.1 130      79 6   15                                         11 2-trans-                                                                          DMAP.sup.3  130 3    1  86  13                                         12                 180      3  52  37                                         __________________________________________________________________________     .sup.1 PAE = methyl pentenoate                                                .sup.2 DABCO = 1,4diazabicyclo[2.2.2]octane                                   .sup.3 DMAP = 4N,N--dimethylaminopyridine                                

EXAMPLE 13

A mixture of 22.8 g of methyl 2-trans-pentenoate and 1.7 g ofN-methylimidazole was heated at 180° C. for one hour in a glassautoclave. After this time, analysis by gas chromatography indicated2.0% of methyl 2-cis-pentenoate, 61.5% of methyl 2-trans-pentenoate and36.3% of methyl 3-cis- and 3-trans-pentenoate. Methyl 3-pentenoate wasthen distilled off continuously from the reaction mixture.

EXAMPLE 14

A mixture of 22.8 g of methyl 2-trans-pentenoate and 2.0 g ofN-methylpiperidine was heated at 180° C. for 4 hours in a glassautoclave. After this time, analysis by gas chromatography indicated1.2% of methyl 2-cis-pentenoate, 72% of methyl 2-trans-pentenoate and26.7% of methyl 3-cis- and trans-pentenoate. Methyl 3-pentenoate wasthen distilled off continuously from the reaction mixture.

COMPARATIVE EXAMPLE 1

A mixture of 11.4 g of methyl 2-cis-pentenoate and 0.79 g of pyridinewas heated at 180° C. for 4 hours in a glass autoclave. After this time,analysis by gas chromatography indicated 97% of methyl 2-cis-pentenoate,0.4% of methyl 2-trans-pentenoate and only 2.5% of methyl 3-cis- andtrans-pentenoate.

COMPARATIVE EXAMPLE 2

As described in Example 1, a mixture of 600 g of methyl crotonate(methyl 2-trans-butenecarboxylate, boiling point 118°-120° C./1,013mbar) and 91 g of 1,8-diazabicyclo[5.4.0]undec-7-ene (BDU) was heated tothe boil in a 1 distillation flask having an attached packed column(height 2 m, diameter 36 mm, 2.1 1 of wire coils) and a reflux divider.A butenecarboxylate mixture having a boiling point of 108°-112° C./1,103mbar was taken off from the top of the column (reflux ratio 10:1).Corresponding amounts of methyl crotonate were metered into the bottomof the column. In the course of 37 hours, 615 g of butenecarboxylatemixture were removed from the top of the column, the said mixtureconsisting of 61% of methyl isocrotonate (methyl2-cis-butenecarboxylate), 30% of methyl crotonate and only 9% of methylvinylacetate.

We claim:
 1. A process for the preparation of a 3-pentenoate byisomerization of a 2-pentenoate to a 3-pentenoate, wherein the2-pentenoate is treated with a tertiary amine having a pK_(a) value>6selected from the group consisting of an aminopyridine of the formula:##STR3## where R₁ and R₂ are identical or different and are each alkylof 1 to 4 carbon atoms, or R₁ and R₂, together with the nitrogen onwhich they are substituents, may form a 5- of 7-membered ring which mayadditionally contain an oxygen or nitrogen atom, and an imidazole of theformula: ##STR4## where R₃ is alkyl of 1 to 4 carbon atoms.
 2. A processas claimed in claim 1, wherein the isomerization is carried out at from100° to 160° C.
 3. A process as claimed in claim 1, wherein from 0.02 to0.2 mole of tertiary amine is used per mole of 2-pentenoate.
 4. Theprocess of claim 1, wherein the tertiary amine is an aminopyridine ofthe formula: ##STR5## where R₁ and R₂ are identical or different and areeach alkyl of 1 to 4 carbon atoms, or R₁ and R₂, together with thenitrogen on which they are substituents, may form a 5- or 7-memberedring which may additionally contain an oxygen or nitrogen atom.
 5. Theprocess of claim 1, wherein the tertiary amine is an imidazole of theformula: ##STR6## where R₃ is alkyl of 1 to 4 carbon atoms.